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Brood parasitism is well-known in its obligate form (e.g. Michener 2007) and little-known in its intraspecific form as an alternative reproductive strategy of pollen collecting bees (e.g. Field 1992). Cleptoparasitic bees with this life strategy are commonly called cuckoo bees.

Obligate cuckoo bees have evolved certainly more than ten times within the bees. Five groups in the family Megachilidae (Rozen 2000), nine possible lineages in the family Halictidae (Michener 1978, 2007), and four independent lineages in the family Apidae (Straka & Bogusch 2007a, Cardinal et al. 2010) have been identified. However, these numbers are highly unstable and changing from one publication to another.

The most common cleptoparasitic strategy is oviposition in the opened still-provisioned brood cells. This strategy occurs in all tribes of the subfamily Nomadinae (Roig-Alsina 1991a) as well as in the tribes Isepeolini (Rozen 2003) and Protepeolini (Roig-Alsina & Rozen 1994) (Apinae). Thus this derived strategy has evolved three times independently in the family Apidae; it is well-known also in some lineages of the family Megachilidae (Rozen 2003). These bees (embedders) generally embed their eggs in the cell wall or cell lining. Embedders use an adaptively modified metasomal apex for careful inletting and positioning of their eggs. Nomadine females have a bilobed sixth sternum with blunt, spinlike bristles, or the sixth sternum might be modified to a simple or bifid pointed apex (Roig-Alsina 1991a). Isepeolini have a similar modification of the sixth sternum with a sharply pointed apex, with or without analogous bristles (Roig-Alsina 1991b), while Protepeolini have an analogously modified sixth metasomal tergum (Roig-Alsina & Rozen 1994). Small eggs of embedders are often totally hidden in the cell wall (except opercula) and have a very complex shape (Rozen 2003). In Biastini, Hexepeolini and Isepeolini, the eggs are inserted parallel to and more or less flush with the wall (Rozen 1992, 2003, Rozen et al. 1997, Rozen et al. in press).

Another strategy of cleptoparasitic bees is oviposition in the fully provisioned nest cells that have already been closed by the host female. This strategy is known in Exaerete, Coelioxoides, Ericrocidini, Melectini, Rhathymini, most Osirini, as well as in cuckoo bees of the families Megachilidae and Halictidae (Rozen 2003, Rozen et al. 2006). These bees (openers) have to open and damage a closure made by host female. They carefully renovate the closure once eggs have been layed. Openers usually have no obvious morphological adaptations of metasoma or egg modifications in comparison with solitary bees.

Epeoloides Giraud (Osirini) seems to be neither an embedder nor an opener. Females of this cleptoparasitic bee may oviposit in opened host cells in the late phase of provisioning and enclose eggs in the cell (encloser); however, behaviour of this genus has not been fully resolved yet (Straka & Bogusch 2007b). This kind of strategy exists also in Melecta albifrons (Förster) as an occasional alternative strategy to cell opening (Straka, unpublished). Behaviour of Epeoloides and Melecta Latreille could be a transitional strategy from openers to embedders. These bees are not the only enclosers. Sphecodes monilicornis (Kirby) females lay eggs in the still-opened cells of their Lasioglossum Curtis hosts. These cells are closed only after Sphecodes Latreille invasion (Sick et al. 1994). Enclosers close off the entire nest tunnel (including the cell) and mask the nest entrance. Host bees are not able to reopen it nor are they able to find the entrance.

Cuckoo bee Epeoloides coecutiens is waiting for the moment, when its host depart, than enter the nest itself.

Epeoloides coecutiens female works in the host nest for one hour, lay an egg to the cell (rigt), and close all the tunel.

A special strategy is known in Sphecodes monilicornis, mentioned above as an encloser. This species must overcome guarding workers in eusocial hosts to reach brood cells. It is not a social parasite and thus it uses a different strategy. The S. monilicornis female is able to kill the guarding female and usually exterminate the entire host nest (Michener 1974, Field 1996, Sick et al. 1994). This species is morphologically adapted for this extermination strategy, with extremely enlarged mandibular muscles and consequently enlarged temporal area. It is the only known species with this strategy, but there are other species with similar morphological characters, but with unknown biology. Table lists newly divided modes of parasitism of cuckoo behaviour. These strategies are partly combinable, but each strategy is associated with its own hallmark behavioural feature. Some of these strategies are evidently plesiomorphic (e.g. cell enclosing) and some derived (e.g. extermination strategy derived from cell enclosing). Hierarchy and relationships among these strategies remain to be studied.

Table: Dividing of brood parasitism in bees to the evolutionary distinct strategies – modes of parasitism